Method for controlling data transmission in a wireless V.24 data transmission system operating between a data terminal and a data transmission device for data telecommunication

ABSTRACT

A method for controlling data transmission in a cordless V.24 data transmission system operated between a data terminal and data communication equipment. The method controls data communication equipment for the purpose of data communication with a first data transmission device connected to the data terminal equipment via a V.24 cable and with a second data transmission device connected to the data communication equipment via a V.24 cable. The first and second data transmission devices can be connected via an air interface by wireless telecommunication. Activating, reactivating and deactivating of an auto-bauding function is reliably performed in such a manner that there is no risk that user data transmission is corrupted until it is deactivated. A state machine contained in the first data transmission device tracks the states of the data communication equipment and acts on the state of the auto-bauding function in the first data transmission device. In the alternative, a state machine contained in the second data transmission device tracks the states of the data communication equipment and acts on the state of the auto-bauding function in the first data transmission device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally pertains to cordless data transmissionsystems. In particular, the present invention pertains to datatransmission in cordless data transmission systems.

2. Discussion of the Related Art

Data telecommunication (datel) is the mutual sending and receiving ofdata or data signals, such as packet data, between a data terminalequipment and a remote data terminal equipment via a telecommunicationnetwork, for example a public telecommunication network, such as ISDN,PSTN, etc. The term data terminal equipment refers to personalcomputers, data terminals, DP systems etc. The term remote data terminalequipment refers to personal computers, data terminals, DP systems etc.So that the data or data signals sent by the data terminal equipment canbe transmitted via the telecommunication network, a network facility,so-called data communication equipment, is provided between the dataterminal equipment and the telecommunication network. Apart from the PCcard, previously known as a PCMCIA card, the most widely used datacommunication equipment is the modem.

The modem is an electrical data communication equipment operating on thebasis of the carrier-current method for use on analogue transmissionpaths of limited bandwidth—e.g. telecommunication lines, such as a/bcircuit pair, ISDN-S₀-BUS etc., of a telecommunication network. Modemsconvert and transmit digital data signals into analogue data signals andvice-versa. Furthermore, a multiplicity of methods of the V seriesstandardized by the International TelecommunicationUnion-Telecommunication Standards (ITU-T)—are implemented in modems.

Discussion of the DECT (Digital Enhanced Cordless Telecommunications)standards is given at pages 1-12 and in the summary inNachrichtentechnik Elektronik 42, January/February 1992, No. 1, Berlin,Del.; in U. Pilger “Struktur des DECT-Standards” (“Structure of the DECTStandard”), pages 23 to 29, in conjunction with ETSI publication ETS300175-1 . . . 9, October 1992; the text of a lecture by A. Elberse, M.Barry and G. Fleming entitled “DECT Data Services—DECT in Fixed andMobile Networks”, on Jun. 17-18, 1996, at the Sofitel Hotel in Paris;the publication by S. Althammer and D. Brückmann entitled“Hochoptimierte IC's für DECT-Schnurlostelefone” (“Highly Optimized ICsfor Cordless DECT Telephones”); and in the DECT publication of the DECTForum, February 1997, pages 1 to 16.

Discussion of the GSM (Groupe Spéciale Mobile or Global System forMobile Communication) standard is given in Informatik Spektrum 14, June1991, No. 3, Berlin, Del.; in A. Mann's “Der GSM-Standard—Grundlage fürdigitale europäische Mobilfunknetze” (“The GSM Standard—Basis forDigital European Mobile Radio Networks”), pages 137 to 152, inconjunction with the publication Telekom Praxis April, 1993; and in P.Smolka's “GSM-Funkschnittstelle—Elemente und Funktionen” (“The GSM RadioInterface—Elements and Functions”), pages 17 to 24.

Discussion of the Universal Mobile Telecommunications System (UMTS) isgiven in Funkschau June 1998; in R. Sietmann “Ringen um dieUMTS-Schnittstelle” (“Fighting for the UMTS Interface”), pages 76 to 81;in WACS or PACS, IS-54, IS-95, PHS, PDC etc.; IEEE CommunicationsMagazine, January 1995, pages 50 to 57; and in D. D. Falconer, et al.,“Time Division Multiple Access Methods for Wireless PersonalCommunications.”

Additional discussion is given in the publication “Vortrag von A.Elberse, M. Barry, G. Fleming zum Thema; in the printed documentsComponents 31 (1993), Vol. 6, pages 215 to 218; and in WO 96/38991.

The basis of the present invention involves controlling, in a cordlessV.24 data transmission system operated between a data terminal equipmentand a data communication equipment for the purpose of datatelecommunication, the reactivating and deactivating of an auto-baudingfunction in such a manner that a reliable implementation of theauto-bauding function becomes possible. According to the method of thepresent invention, there is no risk that the user data transmission,until it is deactivated, is corrupted by the auto-bauding function whichis activated until then.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forcontrolling the reactivating and deactivating of an auto-baud functionin a cordless V.24 data transmission system.

It is another object of the invention to provide a method forcontrolling data transmission in a cordless V.24 data transmissionsystem such that there is no risk that user data transmission will becorrupted by an auto-bauding function.

It is a further object of the invention to provide a method for reliablyimplementing an auto-bauding function for data transmission in acordless V.24 data transmission system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a V.24 data telecommunication system;

FIG. 2 shows a cordless V.24 data transmission system for datatelecommunication;

FIG. 3 shows a cordless V.24 data transmission system according to thepresent invention; and

FIG. 4 shows another cordless V.24 data transmission system according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a V.24 data transmission system is connected to aremote V.24 data transmission system via a public telecommunicationnetwork, for example a PSTN (public switched telecommunication network)exhibiting an a/b circuit pair or an ISDN (integrated services digitalnetwork) exhibiting an ISDN S₀ bus. The V.24 data transmission systemuses a data terminal equipment DTE constructed, such as a personalcomputer and a data communication equipment DCE, such as a modem, whichare connected to one another via a V.24 cable (V.24 interface) K_(V.24).

A remote V.24 data transmission system exhibits a remote data terminalequipment DTE_(r) constructed, such as a personal computer and remotedata communication equipment DCE_(r) constructed, such as a modem whichare also connected to one another via a V.24 cable (V.24 interface)K_(V.24).

The DTE_(r) contains a system controller SST with a use interface BOF,an application software ASW and a driver TR as interface adapter betweensoftware (application software) and hardware, namely data communicationequipment or modem.

The driver TR is specific to the modem manufacturer and preferablyconstructed as a CAPI (common ISDN application programmable interface)driver, such as a standardized communication interface for user softwarefor error-tolerant ISDN telecommunication with a personal computer, oras a TAPI (telephone application programmable interface) driver.

A multiplicity of the data communication equipment DCE which can beobtained on the market, such as analogue modems and ISDN terminaladapters external to the PC are controlled via a HAYES instruction setor HAYES standard. The HAYES standard was originally an Americanindustry standard for modem communication, especially for modem controlby the data terminal equipment DTE. It is also called the AT standardbecause almost all instructions of the HAYES instruction set begin withthe prefix “AT” (Attention) comprising the ASCII characters A and T. Thestandard, which has been introduced throughout the world, is the subjectmatter of an ITU Recommendation (International Telecommunication Union)designated by “ITU-T V.25ter”. Instead of the prefix “AT”, the prefix“at”, the prefix “A/” or the prefix a/” can also be used.

In these modems, the modem driver TR of the data terminal equipment DTEchanges the baud rate at the V.24 interface depending on the datatransmission phase, specifically connection set-up, negotiation of thetransmission parameters etc. This change in baud rate is transmitted viathe V.24 cable K_(V.24) to the data communication equipment or modem,DCE, but not by means of signalling. Thus, the modem DCE mustautomatically detect the newly set baud rate and adapt itself to thedriver TR (auto-bauding). The baud rate at which the “AT” charactersequence is transmitted is valid until the next “AT” character sequenceis received.

This auto-bauding is active in the modem until the modem has changedinto the user data transmission mode, or the so-called “transparentmode” after a connection set-up. In this mode, a “point-to-point”connection (“end-to-end” connection) exists with the remote datacommunication equipment or, the remote modem DCE_(r). Auto-bauding isthen switched off so that “AT” character sequences are not interpretedin the user data stream and there is the possibly that an unwantedchange in baud rate may occur at the modem.

The V.24 cable or V.24 interface K_(V.24) supports modem operation on apersonal computer by means of various lines, often referred to as statuslines, in accordance with ITU-T Specification V.24, March 1993, pages1-19.

These lines are a transmit data line TxD for data transmission, areceive data line RxD for data transmission, an RTS (Ready To Send) lineRTS for the “hardware handshake” type of transmission for transmittingthe “READY TO SEND” state (“RTS” state), a CTS (Clear To Send) line CTSfor the “hardware handshake” type of transmission for transmitting the“CLEAR TO SEND” state (“CTS” state), an RI (Ring Indication) line RI fordetecting ringing at the modem, a DSR (DATA SET READY) line DSR on whichthe modem reports to the personal computer that it is switched on, a DTR(DATA TERMINAL READY) line DTR on which the personal computer reports tothe modem that it is switched on and is ready to accept calls, a DCD(DATA CHANNEL DETECTION) line DCD on which the modem reports to thepersonal computer that it has accepted and set up the connection to aremote modem, and a ground (GrouND) line GND.

If the V.24 cable or V.24 interface K_(V.24) does not have the ninelines listed above but has fewer than nine, e.g. seven, this 7-wirecable can still support the modem operation and the personal computer.This is possible due to the fact that, instead of the “hardwarehandshake” on the RTS/CTS lines, a “software handshake” type oftransmission is performed for transmitting the “RTS”, “CTS” states—e.g.by means of an XON/XOFF protocol—on the transmit/receive data lines TxD,RxD. In the “software handshake”, the data stream transmitted betweenthe data terminal equipment DTE and the data communication equipment DCEis analyzed in the data terminal equipment DTE and the datacommunication equipment DCE, all “software handshake signals” areinterpreted and measures are correspondingly initiated.

For an application scenario in which the data terminal equipment DTE andthe data communication equipment DCE are spatially separated from oneanother, e.g. by several meters, the cord-connected V.24 datatransmission system shown in FIG. 1 exhibits the disadvantage that aV.24 cable K_(V.24), the length of which corresponds to the spatialarrangement of data terminal equipment DTE and data communicationequipment DCE is required for the data telecommunication and, a largeeffort is expended for laying the cable for the installation of thesystem with such cable lengths.

Analogously to cordless telephony, it is therefore desirable and alsoimaginable to replace the cord-connected V.24 data transmission systemaccording to FIG. 1 with a cordless V.24 data transmission system.

Turning now to FIG. 2, which shows a cordless V.24 data transmissionsystem for data telecommunication. A remote V.24 data transmissionsystem (not shown) can either be cord-connected according to FIG. 1 orcordless like the cordless V.24 data transmission system in FIG. 2.

The V.24 cable or V.24 interface K_(V.24) between the data terminalequipment DTE and the data communication equipment DCE are split apartin the cordless V.24 data transmission system and are connected at thetwo ends of the cable caused by the splitting to one data transmissiondevice in each case.

The two data transmission devices, a first data transmission device DTD1connected to the data terminal equipment DTE by the V.24 cable or V.24interface K_(V.24) and a second data transmission device DTD2 connectedto the data communication equipment DCE by the V.24 cable or V.24interface K_(V.24), are connected to one another by an air interface LSSfor wireless telecommunication.

Here interfaces are wireless telecommunication interfaces in whichmessages are transmitted via a remote transmission link between amessage source, such as a first data transmission device DTD1 and amessage sink, such as a second data transmission device DTD2 wirelesslyon the basis of various message transmission methods FDMA (frequencydivision multiple access), TDMA (time division multiple access) and/orCDMA (code division multiple access), such as in accordance with radiostandards, such as DECT.

The air interface LSS provided in FIG. 2 is preferably the DECT airinterface.

DECT technology is suitable for the basic applicability of the DECTtechnology for the wireless mobile remote transmission of voice and/orpacket data in which the user can both become his own network operator,and also have an access capability to a high-level telecommunicationnetwork due to the DECT network access technology with respect to theremote transmission of user data.

If it is intended to replace a V.24 cable link, e.g. between a personalcomputer and a modem, by means of a cordless data transmission linkaccording to FIGS. 1 and 2, it is also necessary to offer the feature of“Auto-bauding” (automatic baud rate recognition at the V.24 interface)normally used in modems at the cordless data transmission link so thatthe modem drivers supplied with a modem can still be used. However, thisauto-bauding is not always active at the modem but must be switched offin the user data transmission mode, the so-called “transparent mode” ofthe modem.

According to the method of the present invention, a cordless V.24 datatransmission system operated between a data terminal equipment, such asa personal computer and data communication equipment, such as a modem,for the purpose of data telecommunication, with a first datatransmission device connected to the data terminal equipment via a V.24cable and with a second data transmission device connected to the datacommunication equipment via a V.24 cable, which is connected via an airinterface by wireless telecommunication, the auto-bauding functionactivated in the first data transmission device and in the datacommunication equipment is deactivated and reactivated at the time atwhich the auto-bauding function would be deactivated and reactivated inthe data communication equipment in a cord-connected V.24 datatransmission system. A state machine contained in the first datatransmission device tracks the states of the data communicationequipment and acts on the state of the auto-bauding in the first datatransmission device or, a state machine contained in the second datatransmission device tracks the states of the data communicationequipment and acts on the state of the auto-bauding in the first datatransmission device.

An embodiment of the present invention will now be explained withreference to FIG. 3, which shows a cordless V.24 data transmissionsystem.

When the data terminal equipment DTE, the data communication equipmentDCE and the V.24 data transmission devices DTD1, DTD2 are switched on,the V.24 data transmission devices DTD1, DTD2, the data terminalequipment DTE and the data communication equipment DCE are operated inan instruction data transmission mode, in which instruction data KD aretransmitted between the data terminal equipment DTE and the datacommunication equipment DCE when the auto-bauding function is activatedin the data communication equipment DCE.

The data communication equipment DCE goes into a user data transmissionmode when the connection set-up to the remote data communicationequipment DCE_(r) is established.

As the data communication equipment DCE changes from the instructiondata transmission mode into the user data transmission mode, the datacommunication equipment DCE deactivates its auto-bauding function andtransmits a first special information item SIF1 to the data terminalequipment DTE to the effect that the connection set-up has taken place.

The first V.24 data transmission device DTD1 monitors the data streamtransmitted from the data communication equipment DCE to the dataterminal equipment DTE, changes from the instruction data transmissionmode to the user data transmission mode when the first specialinformation item SIF1 is detected, and deactivates its auto-baudingfunction before the data terminal equipment DTE in turn changes from theinstruction data transmission mode to the user data transmission mode asit receives the first special information item SIF1 sent by the firstV.24 data transmission device DTD1.

At the end of the user data transmission in the user data transmissionmode, the data terminal equipment DTE transmits an ESCAPE data sequenceEDF in the direction of the data communication equipment DCE and, as aconsequence, changes to the instruction data transmission mode.

The first V.24 data transmission device DTD1 monitors the user datastream transmitted from the data terminal equipment DTE to the datacommunication equipment DCE and, when it detects the ESCAPE datasequence EDF, transmits it by means of signalling SIG to the second V.24data transmission device DTD2, changes from the user data transmissionmode to the instruction data transmission mode and reactivates itsauto-bauding function.

The second V.24 data transmission device DTD2 transmits the ESCAPE datasequence EDF received from the first V.24 data transmission device DTD1to the data communication equipment DCE.

When it detects the ESCAPE data sequence EDF, the data communicationequipment DCE changes from the user data transmission mode to theinstruction data transmission mode and, as a consequence, reactivatesits auto-bauding function.

Another embodiment of the present invention will be explained withreference to FIG. 4, which shows a cordless V.24 data transmissionsystem.

When the data terminal equipment DTE, the data communication equipmentDCE and the V.24 data transmission devices DTD1, DTD2 are switched on,the V.24 data transmission devices DTD1, DTD2, the data terminalequipment DTE and the data communication equipment DCE are operated inan instruction data transmission mode, in which instruction data KD aretransmitted between the data terminal equipment DTE and the datacommunication equipment DCE when the auto-bauding function is activatedin the data communication equipment DCE.

The data communication equipment DCE goes into a user data transmissionmode when the connection set-up to the remote data communicationequipment DCE_(r) is established.

As the data communication equipment DCE changes from the instructiondata transmission mode into the user data transmission mode, the datacommunication equipment DCE deactivates its auto-bauding function andtransmits a first special information item SIF1 to the data terminalequipment DTE to the effect that the connection set-up has taken place.

The second V.24 data transmission device DTD2 monitors the data streamtransmitted from the data communication equipment DCE to the dataterminal equipment DTE and, when it detects the first specialinformation item SIF1, transmits the first special information item anda second special information item SIF2 to the first V.24 datatransmission device DTD1.

When the first V.24 data transmission device DTD1 receives the secondspecial information item SIF2, it changes from the instruction datatransmission mode to the user data transmission mode and deactivates itsauto-bauding function before the data terminal equipment DTE, changesfrom the instruction data transmission mode to the user datatransmission mode as it receives the first special information itemSIF1.

At the end of the user data transmission in the user data transmissionmode, the data terminal equipment DTE transmits an ESCAPE data sequenceEDF in the direction of the data communication equipment DCE and, as aconsequence, changes to the instruction data transmission mode.

The first V.24 data transmission device DTD1 monitors the user datastream transmitted from the data terminal equipment DTE to the datacommunication equipment DCE and, when it detects the ESCAPE datasequence EDF, transmits it by means of signalling SIG to the second V.24data transmission device DTD2.

When it detects the ESCAPE data sequence, the second V.24 datatransmission device transmits a third special information item, namelySIF3, to the first V.24 data transmission device DTD1.

When it receives the third special information item SIF3, the first V.24data transmission device DTD1 changes from the user data transmissionmode to the instruction data transmission mode and, as a consequence,reactivates its auto-bauding function.

The second V.24 data transmission device DTD2 transmits the ESCAPE datasequence EDF received from the first V.24 data transmission device DTD1to the data communication equipment DCE.

When it detects the ESCAPE data sequence EDF, the data communicationequipment DCE changes from the user data transmission mode to theinstruction data transmission mode and reactivates its auto-baudingfunction.

Although modifications and changes may be suggested by those skilled inthe art to which this invention pertains, it is the intention of theinventors to embody within the patent warranted hereon, all changes andmodifications that may be reasonably and properly come under the scopeof their contribution to the art.

1. A method for controlling data transmission in a cordless V.24 datatransmission system operated between data terminal equipment and datacommunication equipment having an auto-bauding function, the V.24 datatransmission system having a first V.24 data transmission device and asecond V.24 data transmission device which are connected to one anothervia an air interface, the first V.24 data transmission device beingconnected to the data terminal equipment, the second V.24 datatransmission device being connected to the data communication equipment,the data communication equipment being connected to a remote datacommunication equipment with downstream remote data terminal equipment,the method comprising the steps of: operating the first V.24 datatransmission device, the second V.24 data transmission device, the dataterminal equipment and the data communication equipment in aninstruction data transmission mode when the data terminal equipment, thedata communication equipment and the V.24 data transmission devices areswitched on, such that instruction data is transmitted between the dataterminal equipment and the data communication equipment when theauto-bauding function is activated in the data communication equipment;operating the data communication equipment in a user data transmissionmode when the connection set-up to the remote data communicationequipment is established; deactivating in the data communicationequipment the auto-bauding function and transmitting a first specialinformation item to the data terminal equipment indicating that theconnection set-up has taken place when the data communication equipmentchanges from the instruction data transmit mode to the user datatransmission mode; monitoring in the first V.24 data transmissiondevice, the data stream transmitted from the data communicationequipment to the data terminal equipment in the first V.24 datatransmission device, before changing in the first V.24 data transmissiondevice from the instruction data transmission mode to the user datatransmission mode, and deactivating in the first V.24 data transmissiondevice the auto-bauding function before the data terminal equipmentchanges from the instruction data transmission mode to the user datatransmission mode as the data terminal equipment receives the firstspecial information item; transmitting from the data terminal equipmentat the end of the user data transmission in the user data transmissionmode, an ESCAPE data sequence in the direction of the data communicationequipment, and changing in the data terminal equipment to theinstruction data transmission mode; monitoring in the first V.24 datatransmission device, the user data stream transmitted from the dataterminal equipment to the data communication equipment, detecting in thefirst V.24 data transmission device the ESCAPE data sequence,transmitting the ESCAPE data sequence to the second V.24 datatransmission device, changing in the first V.24 data transmission devicefrom the user data transmission mode to the instruction datatransmission mode, and reactivating in the data transmission equipmentthe auto-bauding function; transmitting from the second V.24 datatransmission device, the ESCAPE data sequence received from the firstV.24 data transmission device to the data communication equipment; andchanging in the data communication equipment from the user datatransmission mode to the instruction data transmission mode, andreactivating in the data communication equipment the auto-baudingfunction when the ESCAPE data sequence is received.
 2. The method asclaimed in claim 1, wherein the instruction data are Hayes StandardCommand Set-related instructions, including the prefix “AT (attention)”.3. The method as claimed in claim 2, further comprising the step ofusing a modem as the data communication equipment and a personalcomputer as the data terminal equipment.
 4. The method as claimed inclaim 3, further comprising the step of using a DECT as the airinterface.
 5. A method for controlling data transmission in a cordlessV.24 data transmission system operated between data terminal equipmentand data communication equipment for the purpose of datatelecommunication, the V.24 data transmission system having a first V.24data transmission device and a second V.24 data transmission devicewhich are connected to one another via an air interface, the first V.24data transmission device being connected to the data terminal equipment,the second V.24 data transmission device being connected to the datacommunication equipment and the data communication equipment beingconnected to a remote data communication equipment with downstreamremote data terminal equipment, the method comprising the steps of:operating the first V.24 data transmission device, the second V.24 datatransmission device, the data terminal equipment, and the datacommunication equipment in an instruction data transmission mode whenthe data terminal equipment, the data communication equipment and theV.24 data transmission devices are switched on, such that instructiondata is transmitted between the data terminal equipment and the datacommunication equipment when the auto-bauding function is activated inthe data communication equipment; operating the data communicationequipment in a user data transmission mode when the connection set-up tothe remote data communication equipment is established; deactivating inthe data communication equipment the auto-baud function and transmittinga first special information item to the data terminal equipmentindicating that the connection has taken place when the datacommunication equipment changes from the instruction data transmissionmode into the user data transmission mode; monitoring in the second V.24data transmission device the data stream transmitted from the datacommunication equipment to the data terminal equipment and, detecting inthe second V.24 data transmission device the first special informationitem, transmitting from the second V.24 data transmission device to thefirst V.24 data transmission device the first special information itemand a second special information item; when it receives the secondspecial information item, changing in the first V.24 data transmissiondevice when the second special information item is received from theinstruction data transmission mode to the user data transmission mode,and deactivating in the first V.24 data transmission device theauto-bauding function before the data terminal equipment changes fromthe instruction data transmission mode to the user data transmissionmode as the first V.24 data transmission device receives the firstspecial information item; transmitting from the data terminal equipment,an ESCAPE data sequence in the direction of the data communicationequipment, and changing to the instruction data transmission mode at theend of the user data transmission; monitoring in the first V.24 datatransmission device, the user data stream transmitted from the dataterminal equipment to the data communication equipment, and transmittingthe ESCAPE data sequence from the first V.24 data transmission device tothe second V.24 data transmission device when the data terminalequipment detects the ESCAPE data sequence; transmitting from the secondV.24 data transmission device, a third special information item to thefirst V.24 data transmission device when the second V.24 datatransmission device detects the ESCAPE data sequence; changing in thefirst V.24 data transmission device, from the user data transmissionmode to the instruction data transmission mode, and reactivating in thefirst V.24 data transmission device the auto-bauding function when thefirst V.24 data transmission device receives the third specialinformation item; transmitting from the second V.24 data transmissiondevice, to the data communication equipment, the ESCAPE data sequencereceived from the first V.24 data transmission device; and changing inthe data communication equipment, from the user data transmission modeto the instruction data transmission mode, and reactivating in the datacommunication equipment the auto-bauding function when the datacommunication equipment detects the ESCAPE data sequence.
 6. The methodas claimed in claim 5, wherein the instruction data are Hayes StandardCommand Set-related instructions, including the prefix “AT (attention)”.7. The method as claimed in claim 6, further comprising the step ofusing a modem as the data communication equipment and a personalcomputer as the data terminal equipment.
 8. The method as claimed inclaim 7, further comprising the step of using a DECT as the airinterface.